Electrospinning of Ceramic Nanofibers from Polymer PrecursorsBenjamin M. Eick - Purdue Graduate Student
Mr. Eick's Summary:
Unlike previous work using sol-gel chemistry, electrospinning is a fairly well known process, however, this technique has only recently been applied to ceramics. We electrospin polymer precursors that, upon heating, are transformed into ceramics. This new approach to generating ceramic fibers yields orders of magnitude smaller diameter fibers than conventional, industrial, methods. Pre-ceramic polymers offer the ability to create industrially important ceramic fibers of SiC and SiN, which cannot be produced by sol-gel chemistry, as well as SiO2. These materials were chosen for their potential use in armor, high tensile strength, and their refractory properties. Fiber morphology, ceramic yield, and decreasing diameter of fibers are the foci of this research. To this point fibers of less than one micron in diameter have been produced, with the goal of 50 nanometer diameter fibers in the near future.
[This presentation was at a technical level far above this webmaster's understanding - so below is summarized the more non-technical aspects of the presentation]
It would be highly desirable to produce ceramic composite materials that would have the property of transparency. Applications would include windshields for armored vehicles, process view ports, etc. It is known that if ceramic fibers are produced in small enough diameters, light can pass around them (rather than interact) which should make the transparent ceramic composite material feasible.
It is the focus of this project to demonstrate the production of ceramic (particularly SiN and SiC) fibers down in this range of size using the electrospinning process. To use this process requires two components: a precursor for the ceramic material itself, and also a co-spinner polymer to add viscosity to the fluid (which allows formation the fiber geometry as opposed to fine dust.) The process involves placing the precursor ceramic/co-spinner blend into a pipet which is equipped with an electrode to impart a very high positive electrical charge to the blend. A target (at electrical ground) is placed beyond the end of the pipet, and the result is the blend escaping via the only route available (out the pipet hole) forming a lot spider-web like filaments landing on the target. The target of choice for this was a metal washer because the hole allows for easy viewing of the resultant fibers with an optical microscope.
Once the blend is made fibrous by the electrospinning process, it has to be pyrolized by heating in atmosphere to around 1000C for 4 hours, which burns out the polymer, and leaves pure ceramic fibers. There are many variables that can be adjusted. For instance, blend ratios (precursor/polymer), voltage, pipet hole size, pyrolysis conditions, etc.
Using the metal washers, the fibers can readily by evaluated using optical microscopy. Occasionally, Atomic Force Micrographs (AFM) can be produced (at much higher investment in time and expense) to confirm fiber size and geometry with very high clarity images.
To date, fibers of 0.4 micrometer ave. dia. have been achieved. Nice round fibers of 0.13 - 0.47 micrometer diameters have been produced with 20,000 Volts at the anode. Even smaller fibers have been made when the voltage is increased to 40,000 Volts.
There is certainly a long way to go before we see optically clear Kevlar, but the technology does show promise. In just the past year, fiber diameter has been reduced by nearly an order of magnitude.